1. Field
The disclosed embodiments relate to devices used for measuring the adhesion qualities of coatings such as paints, surface protections, adhesive or adhesive films on substrates which they are intended to, or measuring the surface cohesion qualities in general.
More particularly, the disclosed embodiments relate to an improvement of traction pads used by such devices and which are bonded on the surface, the adhesion performances of which are to be measured by exerting a tearing force through said traction pads.
2. Brief Description of Related Developments
Knowing the adhesion qualities of a coating on a substrate, for example paint on its intended support, or the cohesion of the surface of a material, is essential data in industry since the behaviour of parts and assemblies of such parts in their future operating environments greatly depends thereon.
A currently used method for determining such adhesion qualities of a coating or surface cohesion consists in measuring the efforts required for causing the tearing of a sample of the substrate coating on a test-portion, which it has been deposited on, or parts of the surface of the material.
Tests are often carried out on standardised test-portions and follow precisely defined protocols, since being able to reproduce the measures repeatedly in order to be able to have an efficient comparison of the adhesion performances of the various pairs coating-substrate is very useful.
The general principle of such a measurement consists in bonding a rigid pad of the calibrated bonding surface on a sample of the surface to be estimated, i.e. a substrate coated with a coating according to the planned application method or a sample of a material having been exposed to surface treatments and, when the adhesive has set, in applying a traction on the pad until it is torn from the coating of the surface of the substrate or of the surface material.
The force corresponding to such tearing gives us a measure of the qualities of adhesion or cohesion desired.
The standard ISO 4624 describes the procedure to be implemented so that the results obtained by various testing laboratories can be compared.
Tractions devices such as the one described in patent U.S. Pat. No. 3,821,892 can also be found, which are implemented to carry out the tests mentioned.
The described device makes it possible to exert a traction force on a bobbin-shaped bonded pad, and is provided with means for measuring the traction effort applied.
However, although it is clear that the pad is bonded on the tested surface, the standard only mentions that the bonded surface of the pad must be flat. Thus, in the standard ISO 4624, as well as in patent U.S. Pat. No. 3,821,892, nothing mentions that the surface for bonding the pad should be anything but plane, which is a shape which is explicitly requested. When the shape of the traction pad is mentioned, such as its bobbin shape in patent U.S. Pat. No. 3,821,892, the aim is just to provide a good gripping of the pad by the traction means.
Experience shows that the quality of the results obtained upon the implementation of the standard ISO 4624 is closely related to the quality of the bonding of the pad and to the quality of the test preparation. Most of the time, such qualities of preparation and bonding are not totally satisfactory, which results in an important perturbation of the measurements of the tearing forces.
First, because of the fluidity of the adhesive upon the bonding of the pad on the surface to be tested, it is very difficult to check the thickness of the adhesive film and in particular, to have a uniform thickness of such film between the lower surface of the pad and the surface tested. Because of a not-controlled thickness and of the possible side lowering of the pad, the orientation of the axis of the pad can be shifted with respect to the normal to the surface of the test portion, which is the traction direction on the pad during the test. In practice, a deviation, even a small one, of the axis of the pad with respect to the traction axis and a variation of the thickness of the adhesive film at the interface between the traction pad and the tested coating substantially affect the value of the force measured upon the tearing, and this effect is problematic, since it leads to a significant dispersion of measures.
To take this phenomenon in consideration, the test operator is generally obliged to multiply the tests and repeat them a lot of times to obtain average values of the tearing stresses which can be used.
Secondly, a curl of adhesive is formed around the pad upon the bonding of said pad. Such curl is composed of the fluid adhesive which is pushed laterally when the pad is pressed on the surface to be tested to provide a good adherence of the pad. The test procedure provides that the coating submitted to a tearing test is cut along the profile of the pad, so that the surface of the adhesion of the coating deposited on the substrate opposite the pad is taken into account in the measurement of the tearing stresses. A notch is thus cut in the coating around the pad, which requires the previous elimination of the curl of adhesive. Such curl of adhesive, which has hardened at this stage of the operations of the preparation of the test-portion, requires the application of important stresses on the pad itself to be eliminated. Frequently, during such operation of elimination of the adhesive curl, the applied efforts cause the pad to start ungluing, which makes the test portion inappropriate for carrying out the test, or give non-significant results of the test and thus, it is necessary to make a new test portion.